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| United States Patent |
6,024,840
|
|
Rueter
|
February 15, 2000
|
Propylene oxide purification
Abstract
Propylene oxide obtained by an epoxidation process which uses methanol as a
solvent may be effectively treated to remove acetaldehyde by subjecting
the crude epoxidation reaction product to fractional distillation. The
methanol solvent is utilized during such distillation to lower the
relative volatility of the acetaldehyde impurity, thereby making it
possible to obtain a bottoms fraction containing substantially all the
acetaldehyde. Purified propylene oxide having a reduced acetaldehyde
concentration is removed as an overhead stream. Water may also be
effectively separated from the propylene oxide using this procedure.
| Inventors:
|
Rueter; Michael A. (Norristown, PA)
|
| Assignee:
|
Arco Chemical Technology, L.P. (Greenville, DE)
|
| Appl. No.:
|
908604 |
| Filed:
|
August 8, 1997 |
| Current U.S. Class: |
203/50; 203/64; 203/68; 203/74; 203/77; 203/80; 549/531; 549/538 |
| Intern'l Class: |
B01D 003/40; C07C 301/12; C07C 301/32 |
| Field of Search: |
203/18,50,80,74,73,68,77,70,64,29,DIG. 23
549/541,529,531,538
568/913
|
References Cited
U.S. Patent Documents
| 3881996 | May., 1975 | Schmidt | 203/71.
|
| 4140588 | Feb., 1979 | Schmidt | 203/92.
|
| 4349416 | Sep., 1982 | Brandt et al. | 203/19.
|
| 4584063 | Apr., 1986 | Berg et al. | 203/51.
|
| 4597834 | Jul., 1986 | Berg et al. | 203/51.
|
| 4620901 | Nov., 1986 | Berg et al. | 203/51.
|
| 4824976 | Apr., 1989 | Clerici et al. | 549/531.
|
| 4833260 | May., 1989 | Neri et al. | 549/531.
|
| 4971661 | Nov., 1990 | Meyer et al. | 203/54.
|
| 5000825 | Mar., 1991 | Shih et al. | 203/3.
|
| 5133839 | Jul., 1992 | Shih | 203/64.
|
| 5139622 | Aug., 1992 | Marquis et al. | 203/64.
|
| 5274138 | Dec., 1993 | Keating et al. | 549/529.
|
| 5453160 | Sep., 1995 | Peters et al. | 549/541.
|
| 5523426 | Jun., 1996 | Jubin, Jr. et al. | 549/531.
|
| 5591875 | Jan., 1997 | Chang et al. | 549/531.
|
| 5621122 | Apr., 1997 | Saxton et al. | 549/529.
|
| 5646314 | Jul., 1997 | Crocco et al. | 549/531.
|
| Foreign Patent Documents |
| 0732327 | Sep., 1996 | EP.
| |
Primary Examiner: Manoharan; Virginia
Attorney, Agent or Firm: Harper; Stephen D.
Claims
I claim:
1. A method of purifying a crude epoxidation reaction product comprised of
2 to 10 weight percent propylene oxide, 60 to 85 weight percent methanol,
10 to 25 weight percent water and 0.01 to 0.1 weight percent acetaldehyde
comprising the steps of
(a) feeding the crude epoxidation reaction product to a first fractionator
having from 20 to 60 theoretical vapor-liquid contacting stages;
(b) subjecting the crude epoxidation reaction product to fractional
distillation within the first fractionator at a top pressure of 15 to 50
psia, a bottoms temperature of 80.degree. C. to 110.degree. C., and a
reflux:distillate ratio of 10:1 to 30:1;
(c) withdrawing a first overhead stream comprised of propylene oxide and at
least 2 weight percent methanol and having a reduced level of acetaldehyde
as compared to the crude epoxidatimn reaction product from the first
fractionator;
(d) withdrawing a first bottoms stream comprised of methanol, water and at
least 99% of the acetaldehyde present in the crude epoxidation reaction
product from the first fractionator, wherein methanol is present in the
first fractionator at a concentration effective to render the acetaldehyde
less volatile than the propylene oxide.
2. The method of claim 1 wherein the top pressure is 25 to 40 psia, the
bottoms temperature is from 93.degree. C. to 104.degree. C., and the
reflux:distillate ratio is from 15:1 to 25:1.
3. The method of claim 1 wherein the overhead stream is comprised of at
least 4 weight percent methanol.
4. The method of claim 1 wherein the bottoms stream contains at least 99%
of the water present originally in the crude epoxidation reaction product.
5. The method of claim 1 wherein the crude epoxidation reaction product and
the overhead stream both are additionally comprised of propylene and the
overhead stream is fed to a second fractionator wherein the overhead
stream is subjected to distillation such that a second overhead stream
comprised of propylene and a second bottoms stream comprised of propylene
oxide and substantially free of propylene are separately withdrawn from
the second fractionator.
6. The method of claim 5 wherein the second bottoms stream is additionally
comprised of impurities and is fed to an extractive distillation column
wherein the second bottoms stream is subjected to extractive distillation
with an extractive solvent such that a third overhead stream consisting
essentially of propylene oxide and a third bottoms stream comprised of the
extractive solvent and the impurities present in the second bottoms stream
are separately withdrawn from the extractive distillation column.
7. The method of claim 1 wherein the overhead stream is additionally
comprised of impurities and is fed to an extractive distillation column
wherein the overhead stream is subjected to extractive distillation with
an extractive solvent such that a second overhead stream consisting
essentially of propylene oxide and a second bottoms stream comprised of
the extractive solvent and the impurities present in the overhead stream
are separately withdrawn from the extractive distillation column.
8. The method of claim 1 wherein the reflux:distillate ratio is between
15:1 and 25:1.
9. The method of claim 1 wherein the crude epoxidation reaction product has
been obtained from an epoxidation process wherein methanol is used as a
solvent, hydrogen peroxide is used as an oxidant, and a
titanium-containing zeolite is used as a catalyst.
Description
FIELD OF THE INVENTION
This invention provides a method of recovering propylene oxide in purified
form from an epoxidation reaction mixture additionally comprised of
methanol and contaminating amounts of acetaldehyde impurity. Such mixtures
may be formed by epoxidizing propylene with hydrogen peroxide using a
titanium-containing zeolite such as titanium silicalite as a catalyst and
methanol as a reaction solvent. Substantially all of the acetaldehyde may
be removed by fractionation of the epoxidation reaction mixture in a
distillation column wherein a concentration of methanol is maintained in
the distillation column sufficient to suppress the volatility of the
acetaldehyde so as to minimize the amount of acetaldehyde present in the
overhead stream.
BACKGROUND OF THE INVENTION
In recent years, the production of propylene oxide from propylene using
hydrogen peroxide as an oxidant and a titanium-containing zeolite as a
zeolite as a catalyst has been proposed. Methanol is a particularly
preferred reaction solvent for such purposes, as it tends to promote high
catalyst activity and selectivity. Epoxidation processes of this type are
described, for example, in U.S. Pat. Nos. 5,591,875, 4,833,260, 5,621,122,
5,646,314, and 4,824,976, EP Pub. No. 0732327, and Clerici et al., J.
Catalysis 129, 159-167 (1991), the teachings of which are incorporated
herein by reference in their entirety. Although such processes are capable
of providing exceptionally high selectivity to propylene oxide, minor
quantities of certain by-products such as acetaldehyde are inevitably
formed. Since a satisfactory propylene oxide for commercial purposes
should contain less than 100 ppm, and preferably less than 20 ppm,
acetaldehyde, the development of methods for separating substantially all
of the acetaldehyde by-product from such reaction mixtures is necessary.
In addition, epoxidation processes of this type form water as a co-product
with the water being derived from the hydrogen peroxide oxidant. Depending
upon the method used to generate the hydrogen peroxide to be used in the
epoxidation reaction, water may also be present in the feed to the
reactor. While epoxidation processes catalyzed by titanium-containing
zeolites are remarkably tolerant of water, it will be necessary for most
commercial purposes to obtain propylene oxide in substantially anhydrous
form. An efficient method of removing water from the propylene oxide
produced by such an epoxidation process therefore is needed.
SUMMARY OF THE INVENTION
Prior art methods for separating acetaldehyde from propylene oxide, which
were principally developed in connection with organic hydroperoxide-based
epoxidation processes wherein either ethylbenzene or t-butyl alcohol is
used as a reaction solvent, have generally charged the crude propylene
oxide, after removing substantially all the unreacted propylene, to a
distillation column and removed propylene oxide and all lower boiling
materials, including acetaldehyde, as an overhead product. A subsequent
fractionation of the overhead product in a second distillation column then
is employed to separate acetaldehyde as an overhead product from a bottoms
fraction containing the propylene oxide.
We have now found that the substantial quantity of methanol present as a
reaction solvent in a crude epoxidation product from a hydrogen
peroxide/titanium silicalite process may be used to advantage in achieving
the desired separation of acetaldehyde from propylene oxide. The presence
of methanol at high concentrations substantially reduces the volatility of
acetaldehyde relative to propylene oxide. Thus, instead of taking
acetaldehyde overhead as in conventional purification schemes, the present
invention operates by maintaining a concentration of methanol in the
liquid phase within the distillation column sufficiently great so as to
suppress the volatility of acetaldehyde and force all or substantially all
of the acetaldehyde into a bottoms stream. Purified propylene oxide
containing a reduced level of acetaldehyde is taken overhead. The overhead
stream will also have a substantially lower water content as compared to
that of the crude epoxidation reaction product.
More specifically, the present invention provides a method of purifying
propylene oxide produced in an epoxidation process wherein methanol is
used as a solvent comprising the steps of
(a) feeding a crude epoxidation reaction product comprised of propylene
oxide, methanol and acetaldehyde to a fractionate or;
(b) subjecting the crude epoxidation reaction product to fractional
distillation within the fractionator;
(c) withdrawing an overhead stream comprised of propylene oxide and
methanol and having a reduced level of acetaldehyde as compared to the
crude epoxidation reaction product from the fractionator; and
(d) withdrawing a bottoms stream comprised of methanol and acetaldehyde
from the fractionator.
Said fractional distillation is performed under conditions such that the
concentration of methanol within the fractionator is maintained at a level
sufficiently high so as to substantially prevent acetaldehyde from being
present at the point at which the overhead stream is withdrawn from the
fractionator.
DESCRIPTION OF THE DRAWING
The accompanying drawing (FIG. 1) illustrates in schematic form a
particular embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A crude epoxidation reaction product, from which unreacted propylene has
been substantially removed by prior distillation such as a flash
distillation or other such conventional distillation operation, is fed
through line 2 to an intermediate zone of first fractionator 1. Depending
upon the epoxidation conditions employed, the crude epoxidation reaction
product generally has a composition comprised of the following components,
in percent by weight:
______________________________________
propylene oxide 2-10
methanol 60-85
acetaldehyde 0.01-0.1
water 10-25
ring-opened byproducts of
0.1-1
propylene oxide (e.g., propylene glycol)
propylene and/or propane
0.01-0.1
______________________________________
The first fractionator 1 may comprise a conventional distillation column or
tower having a capacity appropriate to handle the desired volume of crude
epoxidation reaction product within a given period of time and having a
sufficient number of theoretical plates to accomplish the necessary
separation of methanol and acetaldehyde from the propylene oxide. The use
of a plate column is particularly advantageous. For economic reasons, the
fractionator should normally comprise a single distillation column or
tower, although the use of multiple columns or towers to accomplish the
same result is not excluded.
Fractionator 1 is operated at conditions such that a first overhead stream
comprised of propylene oxide and a minor amount of methanol can be removed
through line 3 and substantially all of the acetaldehyde (preferably, at
least 99%, more preferably, at least 99.9%) is removed in a first bottoms
stream through line 4. It is essential that such conditions be selected to
provide a concentration of methanol within the first fractionator 1 which
is effective to render the acetaldehyde less volatile than the propylene
oxide. Enough methanol must be taken overhead from the first fractionator
such that a zone exists above the feed tray where both methanol and
propylene oxide concentrations are sufficiently high for acetaldehyde to
be heavier (i.e., less volatile) than both methanol and propylene oxide.
This will prevent acetaldehyde from reaching the top of first fractionator
1 and permits, under optimum conditions, essentially all of the
acetaldehyde in the feed to the first fractionator to be recovered in the
bottoms stream. Typically, the process is operated so as to have at least
2 (more preferably, at least 4) weight percent methanol in the overhead
stream. To minimize the extent of additional downstream processing which
is used, first fractionator 1 is preferably operated at a top pressure of
from about 15 to about 50 psia and at a bottoms temperature of from about
80.degree. C. to about 110.degree. C. It is particularly advantageous to
operate first fractionator 1 such that the top zone thereof is at a
pressure of about 25 to 40 psia and the bottom zone thereof is at a
temperature of about 93.degree. C. to 104.degree. C. First fractionator 1
generally contains from 20 to 60 (more preferably, 30 to 50) theoretical
vapor-liquid contacting stages. A suitable reflux/distillate ratio is
important in achieving optimum results; this reflux ratio should
preferably range between 10:1 and 30:1 (more preferably, 15:1 to 25:1).
Heat to the fractionator may be supplied by means of a reboiler.
The first bottoms stream typically will contain substantial amounts of
methanol and water in addition to the acetaldehyde, together with other
minor by-products such as propylene glycol which have volatilities under
the distillation conditions less than that of propylene oxide. An
additional advantage of the process of this invention is that it is
capable of producing propylene oxide which is substantially free of water,
as the bottoms stream will typically contain nearly all (e.g., at least
99%) of the water originally present in the crude epoxidation reaction
product. The bottoms stream may be purified by conventional means such as
fractional or extractive distillation to recover the methanol for reuse as
the solvent in the epoxidation process.
Where significant amounts of propylene remain in the first overhead stream,
which typically will also contain about 90 to 98 weight percent propylene
oxide, 2 to 6 weight percent methanol, 0 to 2 weight percent propylene,
and less than 20 ppm (more preferably, less than 5 ppm) acetaldehyde, a
second fractionator may be utilized to remove said residual propylene. The
first overhead stream thus may be charged to an intermediate zone of
second fractionator 5 through line 3. Fractionator 5 which is operated so
that the propylene is withdrawn as a second overhead stream from a top
zone through line 6. Propylene oxide which is essentially free of
propylene is withdrawn as a second bottoms fraction from a bottom zone of
second fractionator 5 through line 7. Second fractionator 5 is operated
advantageously at a top zone pressure of from about 140 to about 300 psia
and at a bottom zone temperature of from about 115.degree. C. to about
150.degree. C. Particularly preferred operation involved a pressure of
about 160 to 220 psia in the top zone thereof and a temperature of about
120.degree. C. to 140.degree. C. in the bottom zone thereof. From about 5
to about 15 (more preferably, 5 to 10) theoretical vapor-liquid contacting
stages are preferably present in the second fractionater. The reflux to
distillate ratio is suitably from 10:1 to 50:1 with 20:1 to 40:1 being the
preferred range.
Further purification of the propylene oxide present in the second bottoms
stream may be accomplished if so desired by subjecting said stream to
extractive distillation. Extractive distillation of impure propylene oxide
fractions is well known in the art and is described, for example, in
extensive detail in the following U.S. Pat. Nos., each of which is
incorporated herein by reference in its entirety: 3,337,425, 3,338,800,
3,464,897, 3,578,568, 3,843,488, 4,140,588, 4,971,661, 5,000,825,
5,006,206, 5,116,465, 5,116,466, 5,116,467, 5,129,996, 5,133,839,
5,139,622, 5,145,561, 5,145,563, 5,154,803, 5,154,804, 5,160,587,
5,340,446, 5,453,160, 5,464,505, and 5,620,568.
For example, the second bottoms stream (or, alternatively, the first
overhead stream, particularly where the first overhead stream contains
little or no unreacted propylene) is fed through line 7 to an intermediate
zone of extractive distillation column 8 wherein it is in countercurrent
contact with an extractive solvent such as a hydrocarbon (e.g., octane),
heavy polar organic compound (e.g., propylene glycol), or such other
substance known in the art to be useful for such purpose. Propylene oxide
of high purity is removed from a top zone of column 8 through line 9,
while the extractive solvent containing relatively less volatile
impurities such as water, methanol and the like are removed as an extract
stream from a bottom zone of column 8 through line 10. The extract stream
is fed to an intermediate zone of stripper 11, which is so operated that
components having boiling points above propylene oxide are removed
overhead via line 12. The extractive solvent is removed via line 13 as a
bottoms stream for recycle to an upper zone of extractive distillation
column 8 via line 14. Make-up extractive solvent can be added to line 14
from line 15.
EXAMPLE
In a first distillation column containing 40 theoretical stages (including
reboiler), a crude epoxidation reaction product from which unreacted
propylene is largely removed is introduced at the 16th stage from the top
of the column. A total condenser is used so that distillate product is
withdrawn as a liquid. The feed to the column is as follows:
______________________________________
Component Wt. %
______________________________________
Propylene Oxide 7.2
Water 16.9
Methanol 74.8
Acetaldehyde 0.04
Propylene & Propane
0.06
Ring-Opened Products
0.95
Other Heavy Components
0.05
______________________________________
The first distillation column is operated at a molar reflux ratio (reflux
to distillate) of 19.7. The pressure in the column condenser is set at 30
psia and the column operated with a pressure drop of 0.2 psi per tray,
resulting in a bottom pressure of about 38 psia. The bottoms (reboiler)
temperature is 98.degree. C. and the top (condenser) temperature is
51.degree. C.
Under these conditions, 99.93% of the acetaldehyde in the feed to the
column is recovered in the bottom stream and 99.7% of the propylene oxide
is recovered overhead. The compositions of these two product streams are
as follows:
______________________________________
Distillate
Bottoms
Component Wt. % Wt. %
______________________________________
Propylene Oxide 94.8 0.023
Water 0.0006 18.2
Methanol 4.4 80.5
Acetaldehyde 0.00035 0.044
Propylene & Propane
0.8 0
Ring-Opened Products
0 1.03
Other Heavy Components
0 0.2
______________________________________
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